In operation, a lift rotor of a rotorcraft, e.g. a main lift and propulsion rotor of a helicopter, generates parasitic forces at the head of the lift rotor. The parasitic forces then induce vibration that propagates to the airframe of the rotorcraft, which vibration is particularly perceptible in the cockpit of the rotorcraft.
To mitigate that drawback, devices have been made for attenuating the vibration that is generated, sometimes referred to as rotor head resonators.
In a first type of device, described in particular in document FR 2 749 901, use is made of a flapping weight that is movable along a direction and of return means suitable for repositioning the flapping weight in a predetermined position.
Devices of the first type are effective. Nevertheless, those devices of the first type are sometimes heavy.
Furthermore, since the speed of rotation of the rotor may vary, the frequency of the vibration to be attenuated varies accordingly. Unfortunately, since devices of the first type are designed to attenuate vibration at a given frequency, such devices of the first type are not capable of adapting to frequencies that vary.
It may be observed that document EP 1 007 406 describes a device of the spring-mass type.
In a second type of device, use is made of pendulums that oscillate under the effect of centrifugal force. Each pendulum comprises a heavy element connected to a hinge of a support, the support performing rotary movement about the axis of rotation of the main rotor. For example, the support is provided with a plurality of radial arms forming a star, each radial arm having a hinge connected to a heavy element.
Under such circumstances, and unlike devices of the first type, there is no need to make use of dedicated return means, since centrifugal force provides the required return force.
That feature gives devices of the second type the capacity to adapt automatically to variations in the frequency of the vibration for attenuation.
Indeed, any variation in the speed of rotation of the rotor gives rise not only to a variation in the frequency of the vibration for attenuation, but also to a variation in the centrifugal force exerted on the pendulum. Thus, such devices of the second type are said to be “automatically tuneable” or indeed “automatically adjustable”.
As a function of the nature of the vibration to be damped, the pendulums can oscillate in a first plane perpendicular to the axis of rotation of the main rotor, or indeed in a second plane in which the axis of rotation of the main rotor is inscribed.
Document FR 2 733 483 presents a device of the second type as described above.
In a first variant of a device of the second type, referred to for convenience as a “simple pendular resonator”, the heavy element is a flyweight connected by a link arm to a branch of the support, the support being secured to the hub of the rotor.
The resonant frequency ω of the resonator is given by the following first relationship:
  ω  =      Ω    ⁢                            L          ×          r                                      r            2                    +                                    I              0                        m                              where:                Ω represents the speed of rotation of the support of the device in revolutions per second;        L represents a first distance between the axis of rotation of the main rotor and the hinge of the pendulum;        r represents a second distance between the center of gravity of the heavy element of the pendulum and its hinge to the support;        I0 designates the inertia of the heavy element about the axis of pendular movement of the heavy element;        m represents the mass of the heavy element.        
Although effective, it should be observed that a simple pendular resonator is tuned by adjusting the first distance and/or the second distance.
Consequently, in order to attenuate vibration at high frequencies, e.g. on a helicopter having more than four blades, two solutions may be envisaged.
In the first solution, the first distance needs to be long, which appears to be impossible as a result of the increase in mass and drag that results therefrom.
In the second solution, the second distance needs to be minimized. Nevertheless, that second solution is difficult to apply since it would be appropriate to use hinges that are over-dimensioned in order to support the heavy element.
Document FR 2 018 491 describes a second variant referred to by the person skilled in the art as a “bifilar pendular resonator”, specifically for reducing vibration at high frequencies.
Each bifilar heavy element comprises a U-shaped counterweight which forms a junction jumper engaged astride a branch of a star-shaped support. The counterweight which forms a junction jumper is then provided with two first openings of circular section that co-operate with two second openings of circular section in the corresponding branch via two rollers.
When it is set into movement by centrifugal force, the heavy element moves in circular translation. It is recalled that a body is said to move in circular translation when the body is moving in a plane with two distinct points of the body describing two circular trajectories having the same radius but different centers.
It should be observed that document U.S. Pat. No. 4,212,588 provides for the device to be arranged in a casing provided with partitions.
This second variant of a device of the second type is advantageous in that it makes it possible to reduce the first distance compared with a device made using the first variant.
Nevertheless, in a helicopter provided with a large number of blades, the first distance continues to be long.
Furthermore, it should be observed that the friction between a heavy element and the associated rollers, and between a branch of the support and the rollers associated therewith tends to damage the device as a whole, thereby giving rise to degradation in its performance.
In order to remedy that, an operator needs to lubricate the device for each flight, thereby giving rise to obvious difficulties.
Document FR 2 768 995 presents a third variant of a device of the second type, referred to for convenience as an “accelerated pendular resonator”. According to that document, the heavy element is a flyweight connected by a link arm to a branch of the support, the support not being secured to the hub of the rotor but rather to a drive member rotating at a speed of rotation that is faster than the speed of rotation of the rotor. The resonant frequency ω of the resonator is given by the following first relationship:
  ω  =      Ω    ⁢                            L          ×          r                                      r            2                    +                                    I              0                        m                              where:                Ω represents the speed of rotation of the drive member that is faster than the speed of rotation of the main rotor, given as a number of revolutions per second;        L represents a first distance between the axis of rotation of the main rotor and the hinge of the pendulum;        r represents a second distance between the center of gravity of the heavy element of the pendulum and its hinge to the support;        I0 designates the inertia of the heavy element about the axis of pendular movement of the heavy element;        m represents the mass of the heavy element.        
Compared with the second variant, the first distance is consequently reduced considerably.
However, the device gives rise to non-negligible levels of aerodynamic drag.